Chromatic or optical dispersion is a phenomenon that causes the separation of an optical wave into spectral components with different wavelengths, due to a dependence of the wave's speed on its wavelength. When an optical signal or pulse is launched into an optical communication channel (e.g., optic fiber), its envelope propagates at a group velocity along the communication channel. Since this pulse includes a range of spectral components, each spectral component travels at a slightly different group-velocity, resulting in group-velocity dispersion (“GVD”), intramodal dispersion, or simply fiber dispersion. This separation phenomenon is also commonly referred to as pulse broadening.
As the pulse travels down the optic fiber the spectral components continue to spatially and temporally separate until the pulse is so broad that the difference between a ‘0’ bit and a ‘1’ bit is indistinguishable on the receiving end. As the demand for greater bandwidth increases, the temporal spacing between adjacent bits continues to shrink. If the travel distance is sufficiently large, the leading edge of a pulse can spatially overlap with the trailing edge of a preceding pulse, causing the bits to blur into each other.
With wavelength-division multiplexing (“WDM”) communication systems, chromatic dispersion can be particularly troublesome since this technology multiplexes multiple optical carrier signals, each having a different wavelength, on a single fiber. Since each channel or wavelength is subject to a different amount of dispersion, dispersion compensation techniques must be wavelength dependent.
As optical communication links are upgraded to higher speeds (e.g., metro links upgraded to 10 Gbits/s from 2.5 Gbits/s, new 40 Gbits/s links, etc.) dispersion is becoming the primary technical limitation. Current solutions use dispersion compensated fiber; however, this solution requires vast lengths of fiber, suffers from substantial optical loss, and is not tunable to meet the needs of WDM systems.